High frequency heating method for vapor deposition of coatings onto filaments

ABSTRACT

A method for the pyrolytic deposition of a coating on a filament axially moving through a pyrolytic vapor deposition chamber comprising establishing a radio frequency electric field to heat a selected length of the filament and cause deposition thereon and regulating the density of the electric field to maintain a temperature profile of desired uniformity along the selected length.

Unite States Patent Douglas et al.

1451 May21, 1974 K. Gregory, East Granby; Robert W. Stielau, Portland,all of Conn.

Assignee: United Aircraft Corporation, East Hartford, Conn.

Filed: Feb. 10, 1972 Appl. No.: 225,350

Related US. Application Data Continuation of Ser. No. 865,157, Oct. 9,1969, abandoned.

[56] References Cited UNITED STATES PATENTS 3,410,7l5 ll/l968 Houghll7/23l 3,572,286 3/I97l Forney ll7/l06 3,607,063 9/197] Douglaset al.2l9/l0.6l 3,661,639 5/l972 Caslaw 117/23] Primary Examiner-Alfred L.Leavitt Assistant Examiner-J. Massie Attorney, Agent, or Firm-John D.Del Ponti [5 7] ABSTRACT A method for the pyrolytic deposition of acoating on a filament axially moving through a pyrolytic vapordeposition chamber comprising establishing a radio frequency electricfield to heat a selected length of the Us Cl 117/228 117/933 117/106filament and cause deposition thereon and regulating l t Cl 3 thedensity of the electric field to maintain a temperan t f d d 'f l t l tField of Search 117/106, 107, 9313, 93.2; 2: 2, 6 0 cm um y aong he-seec ed 219/1055, 10.61

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PATENTEU MY 2 1 i974 HIGH FREQUENCY HEATING METHOD FOR VAPOR DEPOSITIONOF COATINGS ONTO FILAMENTS This is a continuation of application Ser.No. 865,157, filed Oct. 9, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to means for treatingfilamentary material and more particularly relates to means forcontinuously heating moving filamentary material by radio frequencyenergy.

In recent years, considerable effort has been expended in thepreparation of low density, high modulus fibers for use as reinforcementmaterials in lightweight composite structures. In particular, filamentsof boron, produced by a method which involves the chemical reduction ofa boron halide onto a direct current resistively heated tungstensubstrate, have found wide application in meeting the stringent demandsof the aerospace industry. A suitable system for vapor depositing boroncoatings by direct current heating is shown for example in copendingU.S. application Ser. No. 618,511 filed Feb. 24, 1967, by Rice andsharing the same assignee as the present application.

Although direct current heating systems have proven effective for someapplications, they have proven limiting or ineffective for others. Inparticular, in continuous processes wherein conductive filaments arepassed through a reaction chamber containing the vaporized coatingmaterial, it is known that during vapor deposition the traversingresistively heated wire changes impedance and causes an unevendissipation of power therein. The resulting temperature variations inthe wire lead to disparate deposition rates along a length of the fiber.In some cases, such as in the deposition of titanium diboride, theimpedance change is so drastic as to cause the fiber to be vaporized. Inother cases, such as in the continuous production of fibers havingelectrically insulating coatings, as for example, coatings of aluminumoxide, electrical contact with the fiber is cut off as soon as theinsulating coating has formed.

Although in recent years a variety of compensating techniques, such asreactor staging or the incorporation of independent heat dissipationmeans, have been relatively successful in minimizing temperaturevariation in certain coating applications, notably boron over tungsten,several unattractive features have remained. It is known for example,that even in those processes wherein quality .composite fibers areproducible in conjunction with the direct current heating mode, thosefibers produced possess attractive characteristics only up to a maximumdiameter. Typically, for example, when boron is vapor deposited on V2mil tungsten to effect a diameter greater than approximately 4 mils, thefiber experiences substantial tensile strengths up to 4 mils withsignificant increasing losses thereafter. Additionally, in a similarprocess wherein boron is deposited on a resistively heated 1 mil carbonmonofilament, a maximum diameter greater than approximately 2.2 mils hasnot been obtainable without a concomitant loss of uniformity and thegeneration of periodic nodes of increased diameter. These nodes appearto be crystalline; and hence, represent weak spots in the filament.

In addition to the limitations on the kinds of coatings and substratesuseable in a DC heating system, as well and toxic effects of mercury, aswell as presenting handling difficulties during filamentary threading ofthe reactor.

SUMMARY OF THE INVENTION The present invention relates to the treatmentof filamentary materials, particularly for pyrolytic depositiontechniques, in an improved process and apparatus whereby filaments canbe heated in substantial disregard of their level of resistivity. Itcontemplates means for exercising close temperature control over a wirewhich is otherwise prone to overheating due to changes in wirecomposition or character.

In accordance with one aspect of this invention a suitable. wire, suchas tungsten, carbon, or the like, is drawn through a reactor containinga decomposable material-containing gas, such as a boron halide admixedwith hydrogen. A selected length of the wire is heated by energy in theradio frequency range to create a hot zone having a temperaturesufficiently high to effect material deposition thereon as it passesthrough the reactor. The RF energy is transmitted to the fiber by an RFenergy coupler which is designed in general, for control of hot Zonecharacteristics and, in particular, to compensate for impedance changesand obtain a desirable power distribution along a selected length of thefiber. The coupler is impedance matched to both the fiber and theoscillator and further, is tuned to the frequency of the oscillator tomaintain a hot zone in the selected length of desired temperatureprofile to effect rapid and uniform deposition on the fiber.

By means of the present invention, a simple and inexpensive process hasbeen discovered which not only overcomes the maximum diameter-fiberdegradation problems associated with prior art deposition processes butalso vastly expands the rangeof materials useable as coatings andsubstrates. The present process has for example, been successful indepositing boron on A mil tungsten to a final diameter of over 15 milswith tensile strengths of 375,000-400,000 psi. Further, boroncarbonfibers having uniform diameters greater than 4.0 (up to 6.8 mils) milshave been produced without the desirable nodal effect. Additionalapplications of the invention include the satisfactory production ofaluminum oxide and titanium diboride fibers. It will be appreciatedthat, as a consequence of the teachings herein, vapor deposition ofmaterials onto a filamentary substrate is achieved in a mannerheretofore unknown.

BRIEF DESCRIPTION OF THE DRAWINGS An understanding of theiinvention willbecome'more apparent to those skilled in the art by reference to thefollowing detailed description when viewed in light of the accompanyingdrawings, wherein:

FIG. 1 is an elevational view, in section, of apparatus employing aquarter wavelength, phased, tuned coaxial cavity coupler; and

FIG. 2 is an elevational view, in section, of apparatus employing amodification of the quarter wavelength tuned coaxial cavity couplershown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,wherein like numerals indicate like parts, an apparatus suitable forcarrying out the method is schematically shown in FIG. 1, which is aview partly'in section.

The apparatus comprises a vertical reaction chamber 10, which may bemade of glass or quartz, fitted at both ends with appropriate closuremeans 12 through which suitable gas inlet 14 and outlet 16 pass forcommunication with the chamber 10. The closure means 12 permit axialpassage ofa wire substrate 18 through the reactor while containingsuitable fluid, such as an inert gas or liquid mercury or the like, toseal it from the atmosphere. Although wire traverse is depicted asdownward in the drawings, direction of movement is not critical andupward movement will produce satisfactory results.

The reaction chamber is received within a quarter-wave phased, tunedcoaxial cavity coupler 20 which actually comprises upper and lowercoaxial cavities 22 and 24 respectively in symmetrical relation. Eachcoupler includes an outer tubular, electrically conductive, enclosure26, a quarter wavelength long, which supports, by annular flange 28, aninner cylindrical inductortube 30, also a quarter wavelength long. Asshown in the drawing, power is fed in by a coaxial lead from a radiofrequency power source 32 capable of generating frequencies within therange of l to 500 megahertz (MHz). A variable capacitor 34 is providedbetween the enclosure 26 and tube 30 for tuning purposes and it will beappreciated that impedance matching may be accomplished by selectiveplacement of the input tap from the power source 32 and proper tuning ofthe variable capacitor 34. The phased coaxial cavity coupler has beenfound to be extremely advantageous in meeting a wide range ofrequirements for various fibers and it is believed that the coaxialcavity concept is primarily responsible for the dramatic resultsachieved since such a configuration is basically independent of thefiber characteristics while producing a high density, axial oscillatingelectric field. By placing two such configurations in line with theproper phasing to obtain a phased, coaxial cavity, there is created ahighly uniform axial electric field distribution of high density.

Tuning of the upper and lower cavities provides a means of varying thespatial distribution of the electric field in the chamber 10 so that hotzones can be obtained and shaped in the coaxially located fiber. Thelength of the cavity section determines the operating frequency whilethe distance between upper and lower cavities determines the length ofthe hot zone to a maximum value dependent on operating frequency andelwtrisdc nsluct v ty h fiber sfiset d- Qf was operated .for borondepositlon with an eleven inch course, since the field is spatiallydistributed, no mechanical contact with the fiber is required to set upthe electric field within the fiber.

As stated hereinbefore, the generation of heat in a fiber by the RFpower depends upon the conversion of electromagnetic energy to thermalenergy through the electrical resistance of the fiber material. Placingthe fiber in a region where an oscillating axial electric field has beenestablished produces oscillating currents in the fiber which dissipatethe RF power as heat according to: power per unit length FR, where I isthe rms value of the current due to the RF electric field, and R is theresistance per unit length of the fiber.

In order to prevent the end heating efiect and thus confine the energyto a selected area in the fiber, the outer end of each outer enclosure26 is provided with a resonant energy trap 36, tunable by a variablecapacitor 38. In essence, the resonators present avery low impedance,thus reflecting escaping energy back into the hot zonef Each cavity 22,24 provides impedance matching between its oscillator 32 and the fiber18 while being tunable to the frequency of the oscillator.

During an investigation of the coupler shown in FIG. I, using a k miltungsten substrate with a gas flow of 2,000 cc/min. (35 percent BCI andpercent H the apparatus was operated for boron deposition with a 28 inchhot zone over which temperature variations were less than i 50C. Data isgiven in the following table:

TABLE I With the same conditions as above, the apparatus by the formerby means of flange 46. Power source 48,

variable capacitor 50 and'end decouplers 52 are located as hereinbeforedescribed.

During aninvestigation of the single cavity system at 148 MHz, using a kmil tungsten substrate with a gas flow of 2,000 cc./min. (35 percent BCI65 percent H a ten inch hot zone was maintained and filamentary boronwas produced according to the following table:

Photomicrographs of etched cross sections of the large diameter fibersproduced using the apparatus of FIG. 2 did not show a cone structure inthe outer portion of the fiber which is typical for comparable boronfibers produced by conventional direct current heating techniques. TheRF heated fiber showed a very uniform boron deposit, which may accountfor the high strengths attained in the large diameter fibers.

It will be appreciated that the procedures abovedescribed are effectiveto transmit the RF power to the fiber for heating purposes. They havebeen designed and tested for impedances varying from to 2,000 ohms perinch. Although no quantitative measurements have been made of efficiencyin terms of power input converted to heat in the fiber, measurementshave been 'made where input and reflected energy are compared.

' tantalum, or metalloids, such as silicon, boron and the like; orcarbon. The chemical compounds used in depositing the above elementswill typically comprise halides, e.g., chlorides, fluorides, iodides, orbromides of the described metals or metalloids.

A reducing gas such as hydrogen will also ordinarily be included as partof the reactant gas or vapor mixture 4 when halides of the metals ormetalloids are employed although inert gases, such as helium, neon,argon, krypton, xenon and the like may also be included, if desired.

Chemical compounds as distinguished from elements, which may bedeposited, include nitrides, carbides, oxides, phosphides, borides andsulfides of such elements as silicon, titanium, zirconium, aluminum andthe like. Typical of such compounds are titanium diboride, titaniumnitride, aluminum oxide, zirconium carbide and the like. To deposit thedescribed chemical compounds, vapors of phosphorous, sulfur, oxygen,carbon, boron, nitrogen and the like, or compounds of such elements,will be included as part of the reactant gas stream.

Aluminum oxide has been deposited on a 1% mil tungsten substrateutilizing aluminum chloride and carbon dioxide with a substratetemperature of 1,200C, and an ambient pressure at less than atmospheric(of the order of 25 inches of water, absolute). Silicon carbide andtitanium diboride were also deposited on V2 mil tungsten substrates.

Any suitable wire or filament whether electrically conductive or not,may be used as a substrate for deposition purposes or simply for heattreatment, as in the case of carbon. For example, the wire or filamentsmay be any of the elements or compounds mentioned above or glass coatedwith the same and the like. Typical substrate filaments may be composedof tungsten, silicon, silicon carbide, boron, carbon, etc.

Certain process modifications are recognized as being useful in thepresent invention. It is recognized for example that multistrandfilaments or a plurality of separate filaments can be simultaneouslypassed through a reactor.

While the present invention has been described with reference toparticular materials, embodiments and operating techniques, it will beunderstood that these examples are illustrative only and thatalternative materials, arrangements and operating conditions than thosealready mentioned will be evident to those skilled in the art.Accordingly, the true scope of the invention will be measured, not bythe illustrative material, but rather in the spirit of the invention, bythe appended claims.

What is claimed is:

1. In those processes for producing composite fibers by vapor depositingmaterial from a materialcontaining decomposable gas on a moving wireheated by electrical power dissipation therein wherein the impedance ofthe wire changes during deposition, the improvement which comprisesmoving said wire sequentially past a plurality of phased electromagneticcouplers each connected to radio frequency source power means toelectromagnetically couple said wire to said radio frequency sourcemeans and thereby to expose the wire to an axial oscillating electricfield having an adjustable density to cause current flow in the wire,said wire having sufficient resistance to dissipate the applied power tocause heating and deposition of said material thereon, and varying thedensity directly with the impedance changes in the wire to maintain apreselected temperature profile therein.

2. A method for the pyrolytic deposition of a coating upon a filamentcomprising:

axially passing the filament through a pyrolytic vapor depositionchamber and at least two coaxial cavity coupling means;

establishing a radio frequency electric field-of variable density insaid deposition chamber by applying radio frequency power through eachof said coupling means to cause current flow in the filament, saidfilament having sufficient resistance to dissipate the applied power tocause heating of a selected length of the filament and depositionthereon as it passes through said deposition chamher; and

regulating the density of said electric field by tuning the pluralcavity coupling means to maintain a substantially uniform temperature.profile along said length.

3. The method according to claim 2 wherein the electric field ofvariable density is established by generating a radio frequency electriccurrent by at least two power source means, each of said power sourcemeans being electrically connected to one of said coaxial cavitycoupling means so as to provide a separate radio frequency electriccurrent to each coupling means.

4. The method according to claim 3 wherein each of said coaxial cavitycoupling means comprises a tuneable coaxial cavity coupler one quarterwavelength long.

5. The method according to claim 4 wherein the density of the. radiofrequency electric field is regulated by varying the distance andphasing between the cavities, impedance matching each coupler to itspower source means and the filament, and tuning each coupler to thefrequency of its power source means.

6. The method according to claim 5 wherein the coating beingpyrolytically deposited is boron and the selected length of filament isheated to within the range of 1,000 to 1,300C i 50C.

7. The method according to claim 6 wherein the filament is tungsten.

2. A method for the pyrolytic deposition of a coating upon a filamentcomprising: axially passing the filament through a pyrolytic vapordeposition chamber and at least two coaxial cavity coupling means;establishing a radio frequency electric field of variable density insaid deposition chamber by applying radio frequency power through eachof said coupling means to cause current flow in the filament, saidfilament having sufficient resistance to dissipate the applied power tocause heating of a selected length of the filament and depositionthereon as it passes through said deposition chamber; and regulating thedensity of said electric field by tuning the plural cavity couplingmeans to maintain a substantially uniform temperature profile along saidlength.
 3. The method according to claim 2 wherein the electric field ofvariable density is established by generating a radio frequency electriccurrent by at least two power source means, each of said power sourcemeans being electrically connected to one of said coaxial cavitycoupling means so as to provide a separate radio frequency electriccurrent to each coupling means.
 4. The method according to claim 3wherein each of said coaxial cavity coupling means comprises a tuneablecoaxial cavity coupler one quarter wavelength long.
 5. The methodaccording to claim 4 wherein the density of the radio frequency electricfield is regulated by varying the distance and phasing between thecavities, impedance matching each coupler to its power source means andthe filament, and tuning each coupler to the frequency of its powersource means.
 6. The method according to claim 5 wherein the coatingbeing pyrolytically deposited is boron and the selected length offilament is heated to within the range of 1,000* to 1,300*C + or - 50*C.7. The method according to claim 6 wherein the filament is tungsten. 8.The method according to claim 6 wherein the filament is carbon.
 9. Themethod of claim 5 wherein the coating being pyrolytically deposited isaluminum oxide and the selected length of filament is heated within therange of 1,000* to 1,300*C + or - 50*C.